The present invention relates to an ignition system for an internal combustion engine in which an exciter coil provided in an electric generator rotating in synchronism with the engine is used as an ignition power source.
A two cycle engine has a potential danger in that unwanted ignition may take place during reverse or backward rotation of the engine crank shaft, with the result that the engine is driven backward by the ignition resulting in self-continuing backward rotation. Various measures have been proposed to prevent ignition during reverse rotation.
One type of known electronically-controlled ignition system comprises
an ignition circuit including an exciter coil for producing a voltage of positive and negative half cycles in synchronism with rotation of the engine, a semiconductor switch which is turned on when an ignition signal is given and an ignition coil having a primay and a secondary windings, the positive hald cycles of the output of the exciter coil being used as the ignition energy and the conduction of the semiconductor switch causing an abrupt change in the primary current of the ignition coil to induce a high voltage for the ignition in the secondary winding of the ignition coil,
a signal coil outputting a maximum advance angle side signal whose absolute value exceeds a threshold level at the maximum advance angle position of the engine and a minimum advance angle side signal whose absolute value exceeds a threshold level at the minimum advance angle position of the engine,
a maximum advance angle side waveform shaping circuit for converting the maximum advance angle side signal into a maximum advance angle position indicating pulse whose duration corresponds to the period for which the maximum advance angle side signal is greater than the threshold level;
a minimum advance angle side waveform shaping circuit for converting the minimum advance angle side signal into a minimum advance angle position indicating pulse whose duration corresponds to the period for which the minimum advance angle side signal is greater than the threshold level, and
an ignition timing control circuit responsive to the maximum advance angle position indicating pulse and the minimum advance angle position indicating pulse for producing the ignition signal used to turn on the semiconductor switch.
To impart backward drive prevention function to such an ignition system, it has been proposed in Japanese Utility Model Application No. 57817/1983, to arrange the exciter coil and the signal coil to have such a phase relationship that the minimum advance angle side signals are produced during the positive half cycles of the exciter coil when the engine is rotating forward and the minimum advance angle side signals are produced during the negative half cycles of the exciter coil when the engine is rotating backward, and to incorporate a bypassing switch circuit for bypassing the minimum advance angle position indicating pulse away from the ignition timing control circuit when the signal coil produces a minimum advance angle side signal during a negative half cycle of the exciter coil, thereby preventing firing action during backward rotation.
In the above-descrived ignition system, the output during the negative half cycles of the exciter coil (the output which does not contribute to the firing action) is short-circuited by a diode, or the output of the negative half cycles of the exciter coil is used to charge a capacitor in a power supply circuit. As a result, the current which flows during the negative half cycles of the exciter coil is larger than the current which flows during the positive half cycles of the exciter coil. By the effect of the armature reaction due to the current which flows during the negative half cycles of the exciter coil, the rise of the positive output of the exciter coil is retarded and hence the period of each positive half cycle becomes shorter. If, therefore, it is so arranged as in the above-descrived proposed system, that the minimum advance angle side signal is generated during the positive half cycles of the exciter coil, the width of a minimum advance angle side signal may become larger than the period of the corresponding positive half cycle of the exciter coil during a high speed rotation of the engine when the minimum advance angle side signal becomes large. In this state, the period of a negative half cycle of the exciter coil and the period of the minimum advance angle side signal may overlap with each other, preventing application of a part of the minimum advance angle position indicating pulse to the ignition timing control circuit. This may shift the ignition timing during the forward rotation, or cause failure of firing action, leading to failure of ignition.